System of photography employing frequency modulation



Nov. l, 1932. l w. T. HOLDEN ETAL 1,885,826

SYSTEM OF PHOTOGRAPHY EMPLQYING FREQUENCY MODULATION Filed Dec. 8, 192s 2 Sheets-Sheet l,

@y Www Nov. l, 1932. w. H. T. HQLDEN ET AL 1,885,826

SYSTEM OF PHOTOGRAPHY EMPLOYING FREQUENCY MODULTIN Filed Dec. 8, 1926 2 Sheets-Sheet 2 UNITED STATES OFFICE WILLIAM H..T.Ho1.n/Eiv;or BitoxIrnNEwxoan. AND RALPH x. BoNELL," or EAST omnn;:nnw"`.rnnsny, Assicnons .To-Annuleer: TELEPHONE AND TELEGEAPH commmconm'g OF'NEW YORK'v 1 ,i v

SYSTEM orrnoroemriimogme 4ianrieimircm imonummer:

Application led December y851,926

This invention relates to alternating current signaling systems, and more particularly to electro-optical transmission systems.

An object of this invention is the maintenance, in an alternating current signal transmission system, of a substantially constant signaling current level at the receiving station irrespective of transmission level variations.

In a preferred embodiment of this invention the signal to be transmitted modulates the frequency of a carrier current` rather than the amplitude of the carrier current,

and at the receivin terminal the incoming frequency modulated wave is ampllfied toa value above a given minimum, taking due accounti of normal variations in the transmission line, and then kept at a substantially constant value by a current .limiting device which is always acting, thereby .producing beyond the current limiting device a signal wave of substantially constant amplitude having its frequency modulated in accordance with a signal such as the tone values' of a picture. This wave of substantially constant amplitude and varyinlgfrequency is then converted into a` wave aving both amplitude and frequency variations, theamplitude being a linear function of the frequency, and thereby made capable of operating a light valve or other ldevice which is sensitive only to the amplitude and substantially independent of the frequency of the operating current.

A more detailed description of the invention follows and is illustrated in the accompanying drawings.

.aiph l oelectric cell 10 is exposed to light va.-

mations' modulated in accordance with the tonevalue of the transparent picture or neg- .ative 11. The transparent picture is fixed upoma transparent drum 12 which is given a motion of rotation and of translation. A small v eleinental area of the picture is illuminated by a source of intense light 13 anda lens system 14 fcuses the light on a small elemental area of the picture. Upon 'passing'l through the picture, the light impinges upon a .pliotoelectric cell 10, thereby causing it to pass current in proportion to the amount of light transmitted through an elemental area o-f thepicture. The movement of the picture iS such that its elemental areas are successively scanned by the beam :of light. The terminals of the photoelectric cell 10 are connected through a sourceof electric current 21 with a substantially distortionless amplifier 20 which, in turn, connects with the frequency modulating apparatus 30.

Theoutput circuit of the amplifier is energized by the battery22 and the circuit is connected to the winding 31 of the input transformer of the frequency modulating apparatus 30. The input transformer of the requency modulating apparatus has three windings, 31V, 32 and 33, the latter two Windings being so connected to the Vacuum tube 35 as to cause it to set up sustained oscillations and provide a carrier current. The oscillating circuit is tuned by the winding 33 andthe condenser 34 and a carrier current of the desired frequency is generated. The input transformer is so constructed and the circuits are so adjusted that the inductance of the winding 33 which determines the frequency of the oscillator will Vary as the inverse square of the current in the'winding 31 which results in the frequency of the oscillations generated by the vacuum tube 35 being a linear function of the input current traversing the winding 31. This current is a linear function of the illumination of the photoelectric cell 10 and by this modulation of the frequency of the oscillator, its frequency output is a linear function of the light density of the picture ceiving apparatus be operated in synchromsm. A suitable means for maintaining such synchronism is disclosed in the patent to Maurice B. Long, No. 1,7 06,032, March 19, 1929, filed December 18, 1923. A LaCour motor whose s eed is controlled by a tuning fork is used fbr driving the apparatus` at the transmitting and at the receiving stations. At the transmitting station the La- Cour motor is geared to the picture drum 12 which is so mounted that it receives a motion of both rotation and translation by any suitable arrangement such as a gear connection to cause rotation and a threaded shaft and xed nut to cause lateral movement. The LaCour motor is supplied with an electric current from any suitable source, such as the battery 61. The speed of operation of this motor is controlled by the tuning fork 62 which, at the end of one of its tines, carries a contact 63 which alternately connects with the contacts 64 'and 65 connected with the two sets of windings on the motor. The

frequency of vibration of the tuning fork controls the speed of the motor. The tuning fork is operated by the battery 66, the magnet 67 andthe contacts 68 and 69 associated with one of the tines of the fork.

A synchronizing circuit is controlled by the contacts 71 and 72 operated by the tuning fork 62. Synchronizing current of suitable frequency and strength is generated by the oscillator and amplif ing apparatus 80, and after passing throng the filter 90 it is impressed upon the transmission line 100 through the repeating coil 101. Both the modulated picture current'and the synchronizing current are simultaneously transmitted over the transmission line 100.

The arrangement of the apparatus at the receivin station is shown in Fig. 2. The incoming requency modulated carrier current from the transmission line 100 passes through the repeating coil 102 to the band filter 110 which allows only the picture currentsto pass to the input amplifier 120. The picture currents are next impressed upon the push-` pull current limiting device 130, such as the vacuum tube arrangement shown on page 374 in the first edition of a book by van der Bil entitled Thermionic Vacuum Tubes, pu lished by McGraw-Hill Book Company,

which is so' adjusted that it limits the amplitude of the output current so that it is always of constant value even when the amplitude of the incoming modulated carrier current is a inimum due to normal causes, such as increased attenuation of the line or low gain in the repeaters. The output of the current limiting device is next passed throuofh a band filter 140 to suppress harmonics of the picture carrier current Which may be produced in the current limiting device. The output of this filter is a variable frequency picture carrier current of substantially unifo'rm amplitude.

They output of the band filter 140 is transmitted to the filter frequency to amplitude converter 15.0 which has a sloping characteristie over the range of the frequencies transmitted such that there is a linear relation be- .tween attenuation and frequency. An arrangement for converting frequency variations to amplitude variations is disclosed in a corp/ending patent application of Maurice B. Long, Serial No. 110,910, filed May 22, 1926. The function of this second filter is/ to convert the output of the first mentioned lter from a uniform amplitude variable frequency current to a current having both amplitude and frequency variation, the amplitude being a linear function of the frequency. In this connection it is important that the filter frequency to amplitude converter be supplied with a current of substantially constant amplitude. If, however, the output of the signal modulating apparatus is such that the relation between frequency and signal strength, the latter in the embodiment here shown being proportional to the illumination of the photoelectric cell, is non-linear, it is still possible to compensate for this nonlinearity by the use of filter circuits having non-linear frequency-transmission characteristics such that the resultant current impressed on the light valve is still a linear function of the illumination of the photoelectric cell. The output now varies in both frequency and amplitude in, accordance with the light variations or tone values of the picture at the transmitting station and can be employed to operate a receiving device such as a light valve which is sensitive to this amplitude but substantially independent of the frequency of the operating current.

The receiving apparatus here shown employs a light valve 160 electrically connected with the output circuit of the high pass lilter 150. The light valve varies the intensity of a beam of light from the light source 161 transmitted through it and through any suitable lenssystem such as the lenses 162 and 163 to properly focus the light beam on the photo-sensitive receiving film 164.

The film 164 is fixed on the surface of the drum 165 which is given a motion of rotation and translation in synchronism with that of izo Lacasse the drum 12 at the transmitting station.` Small elemental areas of theV photo-sensitive film are successively illuminated in accordance with theillumination of similar elemental areas of the picture at the transmitting station andcause a reproduction of the picture in accordance with well-known prinn ciples. The drum 165 is driven by the La- Cour motor 170 supplied vby a source of powcr 171. `The speed o f operation of this motor is controlled bythe tuning fork 172 which has a frequency identical to that of the tuning fork 62 at the transmitting station. A contact 173 is attached at the end of one of the tines of the fork and thiscontact alternatel connects with the contacts 174 and 175, t ereby energizing one or the other of the sets of coils on the LaCou'r motor.

The incoming synchronizing current passes from the transmission line 100 through the repeating coil 102, the filter 190, the amplifier 180 and to the magnet 181 which drives the tuning fork 172. The- LaCour motor 17 0 isl thereby caused to operate in synchronism with the LaCour motor G0 at the transmitting station, in accordance with principles well known in the art, and the picture carrying drums 12 and 165 at the transmitting and receiving stations, respectively, are given synchronous motions of rotation and tra-nslation.

While the apparatus above described embodying the principles of this invention shows a picture transmission system, it is obvious that this invention is applicable to other signal transmitting systems.`

What is claimed is: s

1. In a frequency modulated carrier current signaling system, means at a receiving station for reducing distortion due to fluctuations in amplitude of the transmitted current resulting from variable attenuation properties of the transmission medium, said means comprising a current limiting device of the push-pull type, an amplifier for amplifying the received modulated current of Varying frequency and iiuctuating amphtude, and means for impressing the amplified current upon said current limiting device, said amplifier amplifying the impressed current by such an amount that the ampllfied current is always limited by said current limiting device whatever the amplitude of received current.4

2. In a frequency modulated carrier current signaling system, means at a receiving station for reducing distortion due to fluctuations in amplitude of the transmitted current resulting from variable attenuation properties of the transmission medium, a current limiting device of the push-pull type, an amplifier for amplifying the received modulated current of varying frequency and fluctuating amplitude, and means for 1mpressing the amplified current u on said current limiting device, said ampli er amplifying the impressed current by such an amount that the amplified current is always limited by said current limiting-device whatever the amplitude of received current,

3. In a frequency modulated carrier current signaling system, means at a receiving station for reducing distortion due to iiuctuations in amplitude of the transmitted current resulting from variable attenuation properties of the transmission medium, said means comprising a current limiting device of the push-pull type, an amplifier for amplifying the received modulated current of varying frequency and fluctuating amplitude, means for impressing the amplified current upon said current limiting device, said amplifier amplifying the impressed current by sucli an amount that the amplified current is always limited by said current limiting device whatever the amplitude of received current, and means for eliminating harmonics caused by said current limiting device.

4. A carrier current system comprising means for producing a frequency modulated carrier current whose frequency varies over a considerable range, a transmission line for transmitting said current to a receiving station, said line having varying attenuation properties which introduce fluctuations in said current, an amplifier at a receiving station for, amplifying the received current of varying frequency, and a space discharge current-limiting device of the push-pull type upon which said amplified current is impresse-d, said amplifier amplifying the impressed current by such an amount that the amplified current is limited bysaid current limiting device whatever the amplitude of received current. l

5. An electro-optical transmission system comprising means for generating an oscillating carrier current the frequency of which is vmodulated in accordance with the light tone values of successively scanned elemental areas of a field of view, means at a receiving station for reducing distortion due to iiuctuations in amplitude of the transmitted current resulting from variable attenuation properties of the transmission medium, said means comprising a current limiter, an amplifier for amplifying the received modulated current of varying frequency and fluctuating amplitude, means for impressing the amplified current upon said current limiter, said amplifier amplifying the impressed current by such an vamount that the amplified current is always limited by said current limiter whatever the amplitude of received current, means for eliminating harmonics produced by said current limiter, means controlled by said current of limited amplitude for producing ampli tude modulations corresponding 'to the frequency modulations of the transmitted current, and means controlled by said last mentioned modulations for producing an image of the distant eld of view.

6. A signaling system comprising means for receiving carrier waves the frequency of which is modulated in accordance with a signal, a current limiting device of the pushpull type, means for impressing said Waves upon said current limiting device to produce a corresponding Wave of limited and constant amplitude, and means for utilizing said last mentioned Wave in thegeproduction of the signal.

A signaling system comprising means for receiving a carrier current wave the frequency of which is modulated in accordance with signals, a wave amplitude limiting device of the push-pull type means for impressing said wave upon said limiting device to produce a corresponding Wave of limited and constant amplitude, means for eliminating from said last mentioned wave harmonic components caused by said current limiting device, and means for utilizing the resultant wave in the reproduction of the signal.

In testimony whereof, We have signed our o names to this specication this 7th day of December, 1926.

- WILLIAM H. T. HOLDEN.

RALPH K. BONELL. 

